We organize the properties, compounds, and reactions of Fe, Cu, and Ag, the three most important transition elements. Their refining methods and qualitative ion tests are frequently tested.
Basic Knowledge
Iron Fe: The fourth most abundant metal in Earth’s crust and the metal produced in the largest quantity. Refining in a blast furnace: Fe2O3 + 3CO → 2Fe + 3CO2 (reduction using coke C). Pig iron is converted into steel in a converter by removing carbon. Detection of Fe2+ (pale green) and Fe3+ (yellow-brown):
・Fe2+ + [Fe(CN)6]3- (potassium ferricyanide) → deep-blue precipitate
・Fe3+ + [Fe(CN)6]4- (potassium ferrocyanide) → deep-blue precipitate (both are structurally almost identical: Turnbull’s blue/Prussian blue)
・Fe3+ + SCN- → blood red
Copper Cu: A metal known since ancient times and essential to the electrical and electronics industries. It reaches 99.99% purity by electrorefining. It dissolves in dilute nitric acid with NO production, in concentrated nitric acid with NO2 production, and in hot concentrated sulfuric acid with SO2 production. Cu2+ (blue) + NaOH → Cu(OH)2 (light-blue precipitate), which becomes CuO (black) upon heating.
Silver Ag: Has the highest electrical conductivity. It dissolves in dilute nitric acid: 3Ag + 4HNO3 → 3AgNO3 + NO + 2H2O. Ag+ + Cl- → AgCl (white precipitate), which forms a complex and redissolves in aqueous NH3.
Blast furnace (iron ore + coke + limestone → pig iron)
Converter (pig iron → steel; decarburization by blowing in oxygen)
Prussian blue/Turnbull’s blue (detection of Fe2+/Fe3+; both are based on KFe[Fe(CN)6])
Electrorefining (blister copper → pure copper 99.99%; precious metals in anode slime)
AgNO3 (reduced and turns black in light or with organic matter; photographic sensitizer)
Patina (a corrosion product of copper, basic copper carbonate)
Further Study (Principles and Applications)
Corrosion (rusting) of iron: When O2 and H2O coexist in neutral water, Fe → Fe2+ + 2e- (anode region) and O2 + 2H2O + 4e- → 4OH- (cathode region) proceed as a local cell, producing Fe(OH)2 → Fe(OH)3 → Fe2O3·nH2O (red rust). Rust prevention includes painting, plating, sacrificial anodes (Zn plates), and making stainless steel (passivation by adding Cr).
In electrorefining of copper, precious metals such as Au, Ag, and Pt settle in the anode slime; these by-products are a major source of Japan’s urban mines.
Silver is sensitive to light, and AgCl decomposes in light into Ag + Cl2. This is the principle of silver-salt photography. Aqueous AgNO3 binds to skin and proteins and causes blackening (silver nitrate burns). Historically, Ag was also used as an antimicrobial agent and is still applied in antimicrobial coatings using silver ions.
- Iron refining: blast furnace → converter
- Fe2+ pale green / Fe3+ yellow-brown
- Fe3+ + SCN- = blood red
- Potassium ferricyanide = K3[Fe(CN)6], potassium ferrocyanide = K4[Fe(CN)6]
- Cu dissolves in hot concentrated sulfuric acid and nitric acid (not in dilute sulfuric or hydrochloric acid)
- Precious metals are found in the anode slime from copper electrorefining
- AgCl redissolves in excess NH3 as [Ag(NH3)2]+
Points to Note (Common Confusions and Frequent Errors)
① Potassium ferricyanide K3[Fe(CN)6] (iron(III)) reacts with Fe2+, while potassium ferrocyanide K4[Fe(CN)6] (iron(II)) reacts with Fe3+. The oxidation states of the testing reagent and the reacting ion are opposite. ② Cu(OH)2 does not dissolve in excess NaOH (Cu is not amphoteric), but it does dissolve in excess aqueous NH3 ([Cu(NH3)4]2+). ③ The solubility of AgCl/AgBr/AgI in aqueous NH3 is AgCl>AgBr>>AgI (AgI is almost insoluble).
Practice
- Write the reduction equation for iron in a blast furnace.
- Give three detection methods each for Fe2+ and Fe3+.
- Write the anode and cathode equations for electrorefining copper and state what is contained in the anode slime.